Gangliosides from stimulated murine peritoneal macrophages were separated into monosialo and polysialo factions and the structural character of entities of the polysialo fraction were examined by mass spectrometry. The samples were prepared as permethylated derivatives to evaluate greater structural detaiI and improve sensitivity. Treatment with sialidase prior to NIS studies indicated that the majority of the polysialo fraction had a sialidase-resistant GMla type base with a minor amount of asialoGM1. Detailed structural analyses by periodate oxidation and mass spectrometry indicated a complete absence of any tandem disialo moieties. The major gangliosides in the polysialo fraction consisted of GDla entities comprising IV3-NeuAc,113NeuAc-GgOse4Cer, IV3-NeuGc,113NeuAc-GgOse4Cer, IV3NeuAc,]II3NeuGc-GgOse4Cer, and IV3-NeuGc,H3NeuGc-GgOse4Cer. The minor components consisted of GDIa entities, IV3NeuAc, 1116NeuAcGgOse4Cer, IV3NeuGc, 1116NeuGcGgOse4Cer, and GDla(NeuAc,NeuGc) where the positional isomer(s) could not be determined. Consistent with previous analyses, ceramide portions of all polysialo gangliosides contained solely C18 sphingosine with C16 and C24 fatty acid moieties. These results suggest biosynthesis to proceed along the "a" ganglioside pathway (e.g., GM3-->GM2->Gma->Gdla) and the proposed asialoganglio side or "oc" pathway, asialoGM->GM[lb-+GDl(x. The p sensitive gangliosides appears to be characteristic of murine immune cells.